Surface acoustic wave (SAW) filters, which are small and light, have been used for filters used in mobile communication devices as typified by mobile phones. The SAW filter includes a piezoelectric substrate and an IDT (InterDigital Transducer) located on the piezoelectric substrate, and operates at a frequency depending on a pitch of electrode fingers of the IDT.
In recent years, the transmission rate of the mobile communication device has been increased. Thus, filters operating at a higher frequency have been developed, but the frequency of the SAW filter depends on the electrode finger pitch of the IDT and therefore, there is a limit on decreasing the electrode finger pitch. Thus, it is difficult to meet the requirement for the higher frequency. Thus, piezoelectric thin film resonators have attracted attention. The piezoelectric thin film resonator has a resonance portion in which a lower electrode, a piezoelectric film, and an upper electrode are stacked on a substrate, and the frequency thereof is determined by the thickness of the resonance portion. Thus, it is easy to make the piezoelectric thin film resonator operate at a higher frequency.
An aluminum nitride film is used for the piezoelectric film of the piezoelectric thin film resonator, for example, but aluminum nitride has a piezoelectric constant and an electromechanical coupling coefficient less than those of other piezoelectric materials. To increase the piezoelectric constant, there has been known a technique that adds scandium (Sc) to aluminum nitride and a technique that provides aluminum nitride films having different content rates of Sc between the substrate and an aluminum nitride film to which Sc is added (e.g. Japanese Patent Application Publication No. 2009-10926).
In addition, it has been known that a piezoelectric film with good crystallinity is obtained by forming and thermally treating a first piezoelectric film and then forming a second piezoelectric film on the first piezoelectric film to form the piezoelectric film (e.g. Japanese Patent Application Publication No. 2007-277606). Furthermore, it has been known that a stress is released and good resonance characteristics are obtained in the piezoelectric thin film resonator by forming the piezoelectric film while changing film forming conditions (e.g. Japanese Patent Application Publication No. 2003-60478). It has been known that good temperature characteristics and good resonance characteristics are obtained by stacking a piezoelectric film with a positive temperature coefficient of resonance frequency and a piezoelectric film with a negative temperature coefficient of resonance frequency (e.g. Japanese Patent Application Publication No. 2001-203558).
When an aluminum nitride film containing an additive element is used for the piezoelectric film of the piezoelectric thin film resonator, degradation of characteristics such as a coupling coefficient, a Q-value, and FOM (Figure of Merit: product of the Q-value and the coupling coefficient) is caused by factors such as orientation and film stress of the piezoelectric film and adhesiveness between the piezoelectric film and the electrode.